After the suggestion to add "-filter:v yadif=1,format=yuv420p" everything worked. Here is a log of it:

<L> in other words, is there a good compressed format viable for archival purposes?
<L> good meaning produces high quality with relatively low size
<L> mutt
<L> whoops, that was supposed to be for zsh
<R> L: yes, I would use ffmpeg -i input -c:v libx264 -preset veryslow -c:a flac output.mkv
<R> er, add -crf 18 after the input
<R> that will give you highly compressed (lossy) video and lossless audio
<L> R: darn, i get "Unable to parse option value "-1" as pixel format
<R> pastie.org your command and all output
<L> R: http://pastie.org/5045110
<R> is it interlaced?
<L> i believe it is
<L> i didn't not explicitely deinterlace it
<R> you should, hold on a second
<R> ffmpeg -i input -filter:v yadif:1,format=yuv420p -c:v libx264 -crf 18 -c:a flac output.mkv
<R> L: what is the output fps from the command?
<L> 29.97
<L> i get this tho: No such filter: 'yadif:1'
<R> change it to yadif=1
<R> that should double the framerate of the input.
<L> R: getting somewhere now. must figure out how to set ntsc for v4l2 options
<L> ah, -standard
<L> R: damn, that quality is epic.

So, the question. Can it be implemented in such a way that ffmpeg doesn't just say "Could not find codec parameters", but rather to try to deinterlace it or something, because I believe a lot of ordinary users won't be able to figure out to just add "-filter:v yadif=1,format=yuv420p" out of the blue, without any suggestion either from ffmpeg binary or from reading the docs (which they never do) :)

I have three comments regarding this issue:
The title asks for automatic insertion of the yadif filter. This would be a highly welcome feature but I don't see how it could be achieved: Most DVB streams I know (these are the streams that would profit most from the feature since in Europe most tv stations still send the live recorded / sport material interlaced) signal "interlaced" no matter if the content is actually interlaced or was originally recorded at 24/25fps (movies) and shows no interlacing artefacts whatsoever.

The original report above claims that an input file could not be read with "ffmpeg -i file" but could be read with "ffmpeg -i file -vf xxx". This is simply impossible / would be a very serious bug (but there is no indication this was the case).

For me it was interesting that a file with a "new", unknown fourcc was provided as input to ffmpeg - we try hard to support strange formats even if they appear broken in some way. But this has limits and files produced with MEncoder by the reporter do not fall into this category imo.
(You can choose any fourcc for any material with both MEncoder and ffmpeg but that does not make the resulting file useful.)
Please note that MEncoder does not support remuxing the raw input from tv:// the way you tried, it does support (very well) reencoding v4l input using lavc or another encoder.